In fluid torque converters interposed between an engine and an automatic transmission, a fluid pump is rotatively connected to an engine and a turbine is rotatively connected to an output member. The output member is rotatively connected to driving wheels through the transmission. Pump blades direct transmission fluid against turbine blades such that torque is imparted to the turbine. The fluid passes from the turbine to a stator which redirects the momentum of the fluid back toward the pump. Fluid moving within the torque converter tends to increase in temperature from where it enters the turbine to where it leaves the turbine, particularly when the pump is rotating at a greater speed than the turbine. A portion of the hot fluid exiting the turbine is diverted from the stator for delivery to a cooler. Fluid from the cooler is returned to the converter at the pump inlet. The magnitude of torque transmitted is sufficient to propel the vehicle when the transmission is in gear and the vehicle brake is released.
A condition known as "Gear Engaged Idle" is encountered when the engine is running, the transmission is in gear, and the vehicle is stopped. Typically the vehicle brake is applied to prevent the vehicle from creeping forward. The torque converter turbine is stalled, that is, held motionless, by rotatively locked vehicle wheels. Engine torque must overcome the drag created by the stalled turbine to rotate the pump. This drag torque increases approximately linearly with the speed of the pump. The precise relationship between engine speed and resultant drag torque is a function of the design of the blades of the turbine and the pump. The engine power required to rotate the pump equals the required engine torque multiplied by the engine speed, and therefore increases at near the square of the increase in engine speed. No power is transmitted through the turbine because the turbine is prevented from rotating. The energy expended by the engine is instead largely absorbed by the fluid in the form of heat. This heat absorption significantly increases the cooling needs of the transmission when the idle speed is increased.
Elevated idle speeds are a particular concern with military vehicles. There exists a condition known as "tactical idle" where the vehicle operator selectively boosts idle speed to 50% higher than the normal idle speed. This provides improved vehicle acceleration from a standing stop for combat conditions. The heat absorbed by the fluid may increase by more than 100% with a 50% increase in idle speed. A proportionate increase in cooling capacity would be required. Increasing cooling capacity generally results in an increase in vehicle mass with either the addition of extra heat exchange area, or the addition of engine driven transmission fluid circulating pumps. This extra cooling capacity and its associated weight penalty are only required for the "tactical idle" operating mode.